1. Field of the Invention
This invention generally relates to a device for cancelling ghost and more particularly to a ghost cancelling device (hereunder sometimes referred to as a ghost canceller) for removing ghost or waveform distortion included in video signals inputted in various kinds of video equipment (for example, a television (TV) receiver) for processing TV video signals.
2. Description of the Related art
First, a typical one of conventional ghost cancellers will be described hereinbelow by referring to FIG. 2 (A) which is a schematic block diagram for showing the construction of this conventional ghost canceller 1. In this figure, reference numeral 11 indicates a filtering portion; 12 a weight setting circuit; 13 a waveform extracting circuit; 14 a peak detecting circuit; 15 an arithmetic mean processing circuit; 17 a subtracter; 18 a reference waveform generating circuit; and 36 an amplifier. By employing such a configuration, this conventional ghost canceller removes waveform distortion generated due to ghost interference and so on from video baseband signals.
Next, an operation of the ghost canceller 1 will be described hereinbelow by referring to FIGS. 2 (A) thru 6.
First, digital input video signals (hereunder referred to simply as input video signals) {X.sub.n } coming from a line l.sub.1 are supplied to the filtering portion 11. As shown in FIG. 3, the filtering portion 11 is a filter composed of a finite impulse response (FIR) filter 21, of which the transfer function is denoted by Ga(f) herein, and an infinite impulse response (IIR) filter 22. Further, the IIR filter 22 is composed of an FIR filter 23, of which the transfer function is denoted by Gb(f) herein, and a subtracter 39. Thus, the transfer function G(f) of the filtering portion is given by using the transfer functions Ga(f) and Gb(f) as follows: EQU G(f)=Ga(f)/{1+Gb(f)} (A1).
In addition, the FIR filters 21 and 23 are respectively constructed as shown in FIGS. 4 (A) and (B). That is, as shown in FIG. 4 (A), the FIR filter 21 is a transversal filter comprising a delay block 26 composed of N delay circuits 25 connected in series, a weighting block 37 composed of N+1 weighting circuits 31 each connecting to an input or output terminal of a corresponding one of the delay circuits 25 and an adding and synthesizing circuit 30 to be used for adding and synthesizing an output signal of each of the weighting circuits 31 and a main signal outputted from the delay block 26 through a line l.sub.5. Further, the delay time T of the delay circuit 25 is given by, for example, EQU T=1/4f.sub.sc
where f.sub.sc is the frequency of a chrominance sub-carrier and is nearly equal to 3.58 MHz. When controlling the main signal, the FIR filter 21 operates to control a weight (gain) a.sub.N of the weighting circuit 31, and on the other hand, when controlling removing a pre-ghost and a waveform distortion component prior to the main signal, the filter 21 operates to control the respective weights a.sub.O thru a.sub.N-1 of the weighting circuits 31.
Further, the FIR filter 23 of the IIR filter 22 is, as shown in FIG. 4 (B), a transversal filter comprising a delay block 27 composed of M+1 delay circuits 25 connected in series each of which has the delay time T, a weighting block 38 composed of M+1 weighting circuits 32 each connected to an output or input terminal of a corresponding one of the delay circuits 25 as shown in the figure and an adding and synthesizing circuit 30 for adding and synthesizing output signals of each of the weighting circuits 32. This FIR filter 23 operates to remove a post-ghost or a waveform distortion component posterior to and added to the main signal.
The respective weights of the FIR filters 21 and 23 of the filtering portion 11 are controlled by the weight setting circuit 12. Further, outputs {Y.sub.n } of the filtering portion 11 are drawn from a line l.sub.2 as output video signals and are simultaneously supplied to a waveform extracting circuit 13.
Now, among the input video signals {X.sub.n } applied from the line l.sub.1, are shown in FIG. 5 (A) a sequence of signals of one horizontal scanning interval on which reference signals to be used for detecting waveform distortion such as a ghost are superposed, assuming that the electric potential of a blanking level is set as V.sub.1. In case where all the values of the weights of the filtering portion 11 are zero, the input video signals {X.sub.n } are drawn from the line l.sub.2 as the output video signals {Y.sub.n } without being changed. Incidentally, the time delay of the signals occurring in these circuits due to processing time is omitted herein for simplicity of description. Further, the signals will be treated in similar fashion in the following description.
Referring to FIG. 5 (B), there is shown a section Td of a signal of FIG. 5 (A), which is to be used for detecting the waveform distortion such as a ghost on the basis of a reference signal .rho., by enlarging the section Td in the direction of an axis of abscissa representing time (hereunder referred to as a time axis). The waveform extracting circuit 13 operates to extract this section Td of the signal. Further, there is shown in FIG. 5 (C) the waveform of the signal in case where a ghost .delta. and noises are mixed with the signal of FIG. 5 (B). Such a signal is then supplied to the peak detecting circuit 14 of the next stage. The peak detecting circuit 14 includes a comparator for comparing a level of the input signal with a reference value, a counter for measuring an elapsed time since the starting point of the section Td and a memory or latch circuit for storing the counted value of the position of the peak and so on. In this peak detecting circuit 14, the time at which the signal reaches the maximum value (or the peak value) thereof, that is, the position of the peak in the section Td is detected as 0. Outputs of the peak detecting circuit 14 are supplied to an arithmetic mean processing circuit 15 of the next stage. This arithmetic mean processing circuit 15 includes a memory circuit for storing the signal of the section Td and adders and so on. In this arithmetic mean circuit processing 15, signals, each of which has a waveform as shown in FIG. 5 (C) and is repeatedly inputted every field or frame during a vertical retrace line interval, are added a predetermined number of times in a synchronous manner and then averaged. By such processing of obtaining the arithmetic mean of the signals, noise components having no correlation with the signal can be sufficiently suppressed and thus signals {Y.sub.n '} shown in FIG. 5 (D) are obtained. In case where the inherent spectrum distribution of the reference signal .rho. of FIG. 5 (B) is flat as shown in FIG. 6 (A), the spectrum distribution of the signal including a ghost .delta. as shown in FIG. 5 (D) exhibits a characteristic as shown in FIG. 6 (B) in which fluctuation indicating waveform distortion components generated due to the ghost occurs in the range of frequency from 0 to 4 MHz. Such signals {Y.sub.n '} are supplied to a positive input terminal of the subtracter 17.
The reference waveform generating circuit 18 generates a reference signal {.gamma..sub.n } having a reference waveform as shown in the waveform diagram of FIG. 5 (E) as well as an ideal spectrum distribution as shown in FIG. 6 (A) in synchronization with the position of the peak detected by the peak detecting circuit 14. By subtracting such a reference signal {.gamma..sub.n } from the signal {Y .sub.n '} supplied from the arithmetic mean processing circuit 15 by the subtracter 17, a waveform distortion signal {.epsilon..sub.n } is obtained as shown in FIG. 5 (F). Further, by multiplying the waveform distrotion signal {.epsilon..sub.n } by an appropriate magnification a which is less than 1, an amplifier 36 of the next stage obtains a signal {a .epsilon..sub.n }, which is hereunder represented by {.epsilon..sub.n '}, as shown in FIG. 5 (G). The weight setting circuit 12 detects the position of the peak of the waveform distortion signal {.epsilon..sub.n '}, the width of the waveform distortion in the time axis and the amplitude ratio of the waveform distortion to the peak. Furthermore, the weight setting circuit 12 calculates weights {.omega..sub.n } which can minimum the distortion and then sets such weights at each of the weighting circuit 31 and 32 respectively composing a weighting block 37 and 38 which further composes the FIR filters 21 and 23 of the filtering portion 11. This weight setting circuit 12 is comprised of a microprocessor or a microcomputer and so on from necessity of function of carrying out operations or processing, thereby repeatedly performing calculation and setting of weights {.omega..sub.n }.sub.k (see FIG. 5 (H)) on the basis of the following equation. EQU {.omega..sub.n }.sub.k ={.omega..sub.n }.sub.k-1 -{.epsilon..sub.n '}.sub.k(A 2)
where k satisfies the following inequality 1.ltoreq.k.ltoreq.m in which m denotes the number of repetition of processing to be required to obtain convergence of the value of the weight {.omega..sub.n }.sub.k. Further, in the above equation, n denotes an order number of data in a sequence thereof and {.omega..sub.n }.sub.k denotes the values of weights calculated at k'th time of iteration of processing.
Incidentally, the calculated weights {.omega..sub.n }.sub.k are set in corresponding taps of the transversal filters composing the filtering portion 11.
The method of sequentially updating the detected and inverted component of the waveform distortion as above stated is theoretically correct. The above described algorithm used in the prior art is a kind of Zero Forcing (ZF) Method and performs control of the waveform distortion in proportion to the amplitude thereof. Thus, this method is a preferable control method having the advantage of relatively quick convergence. Furthermore, in case of this conventional method, video signals, from which the waveform distortion such as a ghost is removed, ought be obtained by weighting the taps of the filters of the filtering portion 11 such that the distortion is minimized. Such a conventional apparatus, however, has a problem that if an external disturbance such as a noise enters the apparatus when the estimated weight approaches a true optimum value to some extent, the estimated weight then converges to a stable value different from the true optimum value and on the other hand, in the IIR filter, the estimated weight oscillates or diverges.
Regarding a mean square value of the waveform distortion component data in the section Td, which is to be detected, as a criterion function, as the number of iterative processing increases after the control operation is commenced, the value of the criterion function gradually decreases until reaches a minimal value. Thereafter, the value of the criterion function inversely oscillates and goes away from this minimal value. Further, the value of the criterion minutely oscillates around a value different from the true value to be reached. Alternatively, the value of the criterion function sometimes increases and diverges. Consequently, the conventional method has encountered a serious problem with respect to stability of operations of the conventional apparatus that for example, a ghost cancelling operation thereof is seriously hindered. Thus, the conventional method is impractical.
Turning now to FIG. 2 (B), there is shown another prior art ghost cancelling device (hereunder sometimes referred to as a second prior art ghost canceller). In this figure, reference numeral 11 denotes a filtering portion which is a synthesized filter including FIR and IIR filters; 12 a weight setting circuit; 13 a waveform extracting circuit; 114 a subtracter; 16 a magnification setting circuit; and 18 a reference waveform generating circuit.
Input digital video signals supplied from an input line l.sub.1 are taken out of a line l.sub.2 as output video signals through the filtering portion 11 and are also supplied to the waveform extracting circuit whereupon a portion of the signal corresponding to a predetermined interval (for example, one horizontal scanning interval) including a reference signal is extracted.
Here, a reference waveform of the signal for detecting waveform distortion such as a ghost in the input video signals will be described by referring to FIG. 5. In FIGS. 5 (L) and (M), are shown step-like or rectangular signals superposed on the video signal of a horizontal scanning interval. In case of FIG. 5 (L), a leading edge of the step-like signal is used as a reference signal. Further, in case of FIG. 5 (M), a trailing edge of the step-like signal is used as a reference signal. In these cases, frequency characteristics of the signals in the vicinities of the leading and trailing edges thereof are preliminarily prescribed.
Further, FIG. 5 (N) shows a pulse-like signal superposed on the video signal of a horizontal scanning interval. In this case, frequency characteristic of the signal in the vicinity of the pulse is preliminarily prescribed. Differentiation of the leading and trailing portions of the step-like signals provide pulses as shown in FIG. 5 (N). FIG. 5 (O) shows a vertical synchronization signal of which the trailing portion can be used as a reference signal.
The waveforms of the above described various reference signals are periodically extracted by the waveform extracting circuit 13 and the extracted signal is supplied to a subtracter 141 of the next stage. In the waveform extracting circuit 13, the conversion (for example, differentiation) of the waveform is effected in accordance with the reference signal taken thereinto. Outputs of the waveform extracting circuit 13 are sent to the subtracter 141 whereupon the waveform of the output signal is compared with the inherent waveform (that is, the internal reference waveform) of the reference signal preliminarily calculated in the reference waveform generating circuit 18. The magnification setting circuit 16 establishes a certain magnification on the basis of the results of this comparison. Further, the gains of taps of each of the filters composing the filtering portion 11 are determined in the weight setting circuit 12. The prior art ghost cancelling device is intended to output video signals in which ghosts are cancelled. Incidentally, the weight setting circuit 12 is often constructed by using a microcomputer or microprocessor because of the necessity of a computing function. Further, the description of a delay of the signal generated in the thus constructed signal processing circuit due to the various processing is omitted for convenience of explanation. In the following descriptions of the invention, the delay of the signal will be treated in the same manner.
Further, in this conventional ghost cancelling device (that is, the second prior art ghost canceller), the weights (that is, the gains of the taps of the FIR and IIR filters composing the filtering portion 11) are set on the basis of a signal representing the difference between the reference waveform taken into the waveform extracting circuit 13 and the internal reference waveform calculated in the reference waveform generating circuit 18. At that time, the magnification is constant. Thus, the prior art ghost cancelling device has a problem that for instance, in case where a signal-to-noise ratio (S/N) is small, a false ghost is generated (that is, an erroneous gain of the tap is established) every processing due to noises so that the convergence of the state of the device to a state, in which ghosts are cancelled, becomes slow, that is, it takes long time to settle the device in a state in which the ghosts are cancelled.
Turning to FIG. 2 (C), there is shown still another prior art ghost cancelling device (hereunder sometimes referred to as a third prior art ghost canceller). In this figure, reference numeral 11 indicates a filtering portion (which is a synthesized filter composed of the FIR and IIR filters); 12 a weight setting circuit; 13 a waveform extracting circuit; 14 a peak detecting circuit; 15 an arithmetic mean processing circuit; 116 a waveform converting circuit; 17 a substracter; and 18 a reference waveform generating circuit.
Input digital video signals supplied from an input line l.sub.1 are drawn out of a line l.sub.2 as output video signals through the filtering portion 11 and are also supplied to the waveform extracting circuit 13 whereupon a portion of the signal corresponding to a predetermined interval (for instance, one horizontal scanning interval) including a reference signal is extracted.
Hereinafter, a reference waveform of the signal for detecting waveform distortion such as a ghost in the input video signals will be described by referring to FIG. 5. Similarly as in case of the second prior art ghost canceller, in FIGS. 5 (L) and (M), are shown step-like or rectangular signals superposed on the video signal of a horizontal scanning interval. Further, in case of FIG. 5 (L), a leading edge of the step-like signal is used as a reference signal. Furthermore, in case of FIG. 5 (M), a trailing edge of the step-like signal is used as a reference signal. In these cases, the frequency characteristics of the signals in the vicinities of the leading and trailing edges thereof are preliminarily determined.
Further, FIG. 5 (N) shows a pulse-like signal superposed on the video signal of a horizontal scanning interval. In this case, the frequency characteristic of the signal in the vicinity of the pulse is preliminarily determined. By differentiating the leading and trailing portions of the step-like signals, are obtained pulses as shown in FIG. 5 (N). Moreover, FIG. 5 (O) shows a vertical synchronization signal of which the trailing portion may be also used as a reference signal.
The waveforms of the above described various reference signals are (periodically) extracted by the waveform extracting circuit 13 and the extracted signal is supplied to the peak detecting circuit 14 of the next stage. In the peak detecting circuit 14, the value in the time axis is corrected by detecting a reference time (corresponding to, for instance, the leading or trailing edge of the step-like reference signal or to the peak of the pulse-like reference signal) in a constant interval. That is, the peak detecting circuit 14 is used for preventing malfunction at the time of the arithmetic mean processing and of the comparison of the waveform of a signal outputted from the wave form converting circuit 116 with that of the reference signal effected in the arithmetic mean processing circuit 15. Particularly, the circuit 14 is composed of a comparator for comparing the level of the input signal with the reference value, a counter for measuring a time elapsed since a starting point of a predetermined interval and a memory (or latch) circuit for storing the counted value of the position of the peak.
After the steps of the process up to the operation of the peak detecting circuit are repeated, the output signal of the circuit 14 is supplied to the arithmetic mean processing circuit 15 of the next stage and random noises included in the reference signal portion of the input video signals are reduced. This arithmetic means processing circuit 15 comprises a memory circuit for storing a portion of the inputted signal corresponding to the predetermined interval and adders. The output thereof is supplied to the waveform converting circuit 161 which is comprised of gate circuits, comparators, a memory circuit and a substracter and so on and effects the conversion (for instance, differentiation) of the waveform in accordance with the reference waveform. The reference waveform generating circuit 18 generates the inherent reference waveform (that is, the internal reference waveform) in synchronization with the position of the peak detected by the peak detecting circuit 14. Such an internal reference signal is compared with or substracted from the output of the waveform converting circuit 161 (that is, a reference signal extracted from the input signal) by the waveform comparing circuit 17. On the basis of the result of the comparison (for example, the difference between them), the waveform of the output of the waveform comparing circuit 17 is converted (by, for instance, multiplying the output thereof by a certain magnification and adding the result to the current gains of the taps) and the gains of the taps of the filtering portion 11 are determined. This prior art ghost cancelling device is intended to output video signals in which ghosts are cancelled. Further, the weight setting circuit 12 may be constructed by using a microcomputer or microprocessor because of the necessity of a computing function. Incidentally, the description of a delay of the signal generated in the thus constructed signal processing circuit due to the various processing is omitted for convenience of explanation.
Meanwhile, in this ghost canceller, there are two types of methods of controlling the determination of the gains of taps. One of them is a feedback type control method by which the extraction of the reference signal is effected at the output side of the filtering portion 11 through a line l.sub.4 and further the gains of the taps are serially updated. The other is a feedforward type method by which the extraction of the reference signal is effected at the input side of the filtering portion 11 through a line l.sub.3 and further the determination of the gains of the taps is not based on the outputs of the filters obtained by using the previously determined gains of the taps. However, in any case of using these methods, random noises components included in the reference signal portions of the input video signals are reduced by the arithmetic mean processing circuit.
Furthermore, in order to prevent the generation of the false ghost, that is, the erroneous setting of the weight data when the weights are set (that is, the gains of the taps of the FIR and IIR filters composing the filtering portion 11), it is necessary to perform adding and averaging operations of more than a considerable number every calculation of data. Thus, this conventional ghost cancelling device has a problem that it takes time to calculate the weight data at a time and the value of S/N obtained as the results of the total arithmetic mean processing is not more than that of S/N obtained as the result of performing the arithmetic mean processing one time.
The present invention is accomplished to resolve the above described problems of the prior art.